In an infrared sensor, a sensing element (sensing section) absorbs infrared rays emitted from an object to raise the temperature of itself, and temperature information of the object is obtained by detecting the change in characteristics of the sensing element according to the temperature rise.
For describing an example of a conventional bolometer-type infrared sensor, a schematic perspective view of a sensing element section constituting the sensor is shown in FIG. 10. The bolometer-type infrared sensor is a kind of a heat-type infrared sensor that senses the temperature information of an object by converting the resistance change caused by the temperature rise of a resistor constituting the sensor, into electric current change, voltage change, or the like. In the Figure, the numeral 101 represents a resistor made of a material having a large rate of resistance change with temperature. A heat insulating structure 104 (void section) is formed on a silicon substrate 103 by micromachining to effectively raise the temperature of the resistor 101 and to maintain the raised temperature of the sensing element. The numeral 102 represents an electrode wiring for taking signals out from the sensing element to the outside.
As a heat-type infrared sensor, those utilizing the change in forward voltage of a semiconductor junction diode are also proposed. FIG. 11 shows a circuit diagram for describing an infrared sensor of junction diode type. The numeral 105 represents junction diodes (here, four diodes are connected), and a constant electric current flows through these diodes 105 from a constant electric current source 106. When infrared rays (IR) from an object are radiated onto the diodes 105, the forward voltage of the diodes change. Accordingly, temperature information of the object can be obtained by sensing the amount of change thereof by a signal output line 107. Since the amount of change in the forward voltage is smaller than that of a bolometer type that uses a material having a large resistance-changing rate, the junction-type sensors are inferior in sensitivity. However, the junction type sensor has an advantage in that it can be fabricated together with the signal reading circuit by using silicon IC processing. Also, the junction type sensor has an advantage that it has little instability of characteristics and little non-uniformity within a wafer surface; the bolometer type tends to have these problems. Also, the problem of low sensitivity of the junction type can be improved by connecting a plurality of diodes.
An infrared sensor array obtained by two-dimensionally arranging a plurality of the above-described heat-type infrared sensors is utilized as a solid image sensor for a dark field observation camera and others. The efficiency of an infrared sensor array is represented by a noise equivalent temperature difference (hereafter referred to as NETD). This is a ratio of the noise in signals between the sensors to the temperature sensitivity. For an improvement of NETD, the control of the noise is important as well as the enlargement of the temperature sensitivity.
The noise is generated in the sensor (or in the sensing element that constitute the sensor) and the reading circuit. By limiting an unnecessary signal band, the noise can be effectively controlled. A representative method of limiting the signal band is an integration circuit. FIG. 12 represents an example of the method in which a gate modulation circuit (a kind of integration circuits) is used for a quantum-type infrared sensor. In the Figure, the numeral 111 represents a quantum-type sensing element. When the infrared light enters, the carriers generated in the sensing element 111 are transmitted through a load resistance 112 to change the voltage of a node 113. An electric capacitor 114 has been charged to a certain voltage in advance by a reset switch 115. The change at the node 113 gives rise to change in the electric current of a MOSFET 117, thereby altering the amount of electric current that is discharged from the electric capacitor 114. Accordingly, the electric current value of the signal output line 116 after a predetermined discharging time depends on the amount of carriers generated in the sensing element 111, i.e. the amount of the incident light. The amount of change in the electric current value of the signal output line 116 is determined by the amount of voltage change in the node 113, the magnitude of the electric capacitor 114, the period, of discharging time, and the mutual conductance of the MOSFET 117. The period of time of electric discharge determines the signal band. The longer the discharge period, the more the band is limited to reduce the noise.
As described above, in a quantum-type sensor, the noise can be suppressed by using a of gate modulation circuit. However, if such a technique is applied to a heat-type sensor, as the output variation greatly varies when the temperature of the sensor changes, a problem rises that a small temperature change caused by the incident infrared rays, which is an aim of the sensing, cannot be read out.
The present invention has been made in order to solve the above-mentioned problems, and to provide a heat-type infrared sensor in which the noise controlling technique used in the quantum-type sensor can be applied. In other words, it is an object of the present invention to provide an infrared sensor wherein the output variations caused by the temperature change in the sensor can be suppressed by sensing the temperature of the whole sensor. It is another object of the present invention to provide an infrared sensor array in which such infrared sensors are arranged in a one-dimensional or two-dimensional array.